The recent increase of information amount communicated over communication lines demands the increasing a bit rate per single wavelength in optical communication networks. However, in order to transmit a signal by a single wavelength through a high-speed line at 40 Gbit/second, and 100 Gbit/second, etc., it is difficult to obtain sufficient transmissive characteristics even though a conventional intensity modulation/direct detection scheme is adopted. Consequently, a new digital coherent scheme is adopted. According to the digital coherent scheme, a local oscillation light source that is unnecessary in the case of the direct detection scheme is provided inside a coherent optical receiver. It is extremely important to control the wavelength of local oscillated light output by that local oscillation light source to match the wavelength of transmitted signal light or to be within predetermined wavelength difference range thereof.
For example, in the case of homodyne detection, respective wavelengths of the signal light and the local oscillated light need to be controlled so as to be substantially same. In general, an appropriate signal processing become unable when the wavelength difference between the signal light and the local oscillated light is out of the range within approximately ±10 pm. Moreover, the load of the signal processing for detecting a wavelength mismatch is large. Consequently, it is desirable that a wavelength relationship between the signal light and the local oscillated light should be stable as much as possible. However, even if lasers which are controlled to be extremely stable are used as the light sources of the signal light and the local oscillated light in general, a wavelength variation of approximately ±20 pm occurs at each light source. Accordingly, when both wavelength variations are combined, the wavelength variation of ±40 pm occurs at a maximum.
Because of the above-explained circumstance, a technology for highly precise control of the wavelength of light output by the local oscillation light source provided in the coherent optical receiver is proposed. For example, Unexamined Japanese Patent Application KOKAI Publication No. H02-307027 discloses a measurement device for highly precise measurement of the frequency of signal light. The measurement device measures the frequency of signal light based on the frequency of an optical signal that is obtained by synthesizing light (local oscillated light) output by a frequency-variable light source with light (signal light) output by a measurement-target light source, and the frequency of local oscillated light. By using such a measurement device, it becomes possible to adjust the wavelength of the local oscillated light to follow the wavelength of the signal light.
However, the forgoing measurement device uses a fabry-perot etalon resonator. Such a resonator needs a predetermined size in order to prevent interference of lights output by the frequency-variable light source and by a frequency reference light source. Consequently, it is necessary to increase the size of the coherent optical receiver having the measurement device. Moreover, it is necessary to set a large resonator to be horizontal in order to execute highly precise frequency control.